Carbon and oxygen isotopic compositions of the Dengying Formation Maidiping Member of Upper Sinian and Sinian-Cambrian boundary division in Ningnan area, western Panzhihua, Sichuan, China.

This paper has discussed composition characteristics of carbon and oxygen isotopes, and discussed Sinian-Cambrian boundary of the area according to the δ13C content variation in the profile. The results showed that the δ13C values of the Maidiping bed dolostone are between -3.19‰~2.86‰ with an average value of 0.10‰; the δ18O values are between -10.75‰~-5.17‰ with an average value of -7.82‰. Within the profile, the δ13C values follow a certain law. At the bottom of the profile, from 2 to 21 bed, all δ13C values are positive, between 0.04‰~1.68‰, with a calculated average value of 0.92‰. At the bottom of the profile, form 23 to 32 bed，all δ13C values are negative, between -3.19‰~-1.42‰ with a calculated average of -2.13‰. There is a significant negative drift, and the variation can be as much as +4‰.The δ13C variation tendency of the Maidiping Member dolostone is similar to those of the strata during the same period worldwide. Therefore it can be used as geochemistry symbol for defining the Sinian-Cambrian boundary.